Two major HIV issues confront clinicians and investigators: (1) ongoing endemic transmission of HIV; and (2) failure to reach an important portion of the HIV-infected population with HAART. Pilot studies, and an ongoing intervention in Atlanta at the Grady Health System (which includes Grady Hospital and the Infectious Disease Clinic, a major provider of HIV care) suggest the presence of a substantial group of people, perhaps equal to or greater in size than the current clinic population, who have characteristics that link these two HIV issues: they use only the hospital and receive no outpatient care; their social and sexual networks have a high prevalence of HIV (64 percent, compared to 19 percent among the contacts of clinic attenders); many have received episodic, inadequate antiretroviral therapy; and they are more intense drug users than are those attending the clinic. We posit that this group, identifiable through their health care seeking behavior, may be critical for both ongoing endemic transmission of HIV, and are the persons least reachable for systematic HIV care and HAART. In order to define and describe this group, and collect information that can lead to interventions that can interrupt transmission and provide HAART to those requiring it, we propose to a longitudinal study of three groups of persons (60 in each group): Nonattenders (those who never use clinic services); Regular Attenders (those who regularly use the clinic); and Intermittent Attenders (those with irregular attendance patterns). We will investigate the demographic, socioeconomic, behavioral, psychological and social network characteristics of these persons (specific aim 1.0). We will use clinical data, contact evaluation, and social network information to determine the potential contribution of these groups to HIV transmission (specific aim 2.0). We will assess the psychosocial receptivity of Nonattenders to systematic HIV care using a standard psychologic instrument (the BSI) and will use the Transtheoretical Model to identify their readiness to commit to care; and we will compare the level of genomic resistance to HIV in the three groups to assess the degree, if any, to which viral resistance may be important in current endemic transmission, and may influence the potential treatment of Nonattenders (specific aim 3.0). These studies will lead to a better understanding of the interaction of community endemicity of HIV and treatment with HAART, set the stage for interventions to bring HAART to those not currently receiving it, and provide a basis, in future work for an understanding of the impact of HAART on endemic transmission